Simple threaded fasteners include a male threaded component and a female threaded component configured to engage one with another. According to the Industrial Fasteners Institute (IFI), a thread is defined as a uniform section in the form of a helix on the external or internal surface of a cylinder. To improve the performance of threaded fasteners for different purposes, a variety of thread configurations are known. In general, threads of all types are based on a straight-line helix pattern. The thread pitch may vary from one type or size of fastener to another, and it is known to provide a differing thread pitch on different portions of the same fastener. However, regardless of the pitch the thread follows a straight-line helical pattern.
In a standard fastener design, for a given or “basic” pitch, the male thread is provided at equal to or less than the basic pitch and the female thread is provided at equal to or more than the basic pitch. The result is that the male thread “floats” within the female thread, allowing the two components to be run together throughout the thread length with little or no interference until clamping pressure is applied during final tightening of the fastener. As the fastener is tightened and clamp load applied, friction is created from stretch in the fastener as it is placed under tension. Clamp load can loosen from vibration, slip of the angular thread surfaces over time, expansion and contraction cycles and the like. It is known to provide prevailing torque by deforming the nut or using paste-like substances in the threads to maintain the relative position of the male and female components, even if clamp load is lost. Crimping a nut adds considerable cost to the manufacturing process, and known paste-like substances for the purpose are somewhat limited in extreme temperature conditions and have limited reusability.
Further difficulties occur when threaded fasteners are used in somewhat non-standard situations. Tightening the fastener requires application of clamp load, meaning the head of the fastener, the confronting surface of the female fastener and all materials or components therebetween are compressed together. In so called “soft joints” such as, for example, joints holding together elastic materials, gaskets or the like, it is desirable that the fastener joint be tight without excessive clamp force applied on the material being held. Shoulder bolts or standard fasteners with spacers have been used for this purpose, complicating assembly and increasing costs.
Threaded fasteners are known to be preassembled in components that are designed to be installed with other components or associated members, and thereafter tightened. For example, various electrical assemblies are provided for use in the field with screws already in place on terminals to receive wires therein. With the wire properly positioned, the screw is tightened to establish electrical connection between the wire and the electrical assembly. The pre-installed position of the screw must be relatively secure so that the screw does not become lost, making the component unusable.
U.S. Pat. No. 7,326,014 discloses an interactive fit screw thread that includes a curved line path thread provided within the helical thread pattern on the shank of a fastener. The curved thread pitch is provided to establish prevailing torque along desired portions of the fastener. Instead of the generally straight-line helical path followed by common fastener threads, the interactive fit screw disclosed in the '014 patent follows a curved-line path in a helical pattern. The exemplary embodiment disclosed in the '014 patent follows a sinusoidal wave path.
To assure aligned attachment with a mating thread, three wave periods covering 360° around the fastener shank are used in the embodiments disclosed in the aforementioned U.S. Pat. No. 7,326,014. Each wave crest is 120° from the previous wave crest, and the waves follow immediately after one another so that the waves are clustered on a single thread pitch. This works effectively; however, to achieve the advantages provided from the wave portions of the thread, the fastener is position dependent. In a threaded assembly, it is necessary to have the wave thread portion properly located with respect to the mating thread to ensure proper engagement and alignment of three waves. For example, it is common to provide four internal thread pitches in a female fastener component. To work most effectively, three waves of the male fastener component should be located in engagement with the threads of the female component. If the male fastener component is over rotated or under rotated relative to the female fastener component, so that less than three waves are engaged with the female component, the threaded assembly can be less effective. Accordingly, it has been necessary to have a coordinated knowledge of the final fastener component relationships so that the waves can be positioned appropriately for finally engagement in the female threaded component.
It is desirable to provide a threaded fastener having an interactive fit screw thread that is not position dependent and that provides prevailing torque along an elongated portion of the thread length.